Flux-cored wire for gas shielded welding use is spreading widely throughout the industrial world as a welding material which enables use of a high efficiency welding method, but the majority of this is flux-cored wire having Ti oxides such as rutile as the main slag-forming material (below, sometimes abbreviated as “rutile-type FCW”).
A rutile-type FCW has excellent welding work efficiency and has the feature of enable easy welding at flat, vertical, overhead, horizontal, and other various welding positions. However, sometimes the high content of oxygen of the weld metal becomes a problem. That is, when using a rutile-type FCW, in weld metal of high strength steel or low temperature use steel, securing the necessary extent of toughness often becomes difficult. The facts that this is caused by the oxygen in the weld metal being present as non-metal inclusions and that these inclusions become the initiation points of cracks has been well known from previous research works. To solve this problem, a rutile-type FCW where reduction of the amount of oxygen of the weld metal is attempted has also been studied up to now. The specific results of the studies are disclosed in for example PLT's 1 to 3 etc.
That is, in PLT 1, the strongly deoxidizing elements Ca and Al are added to reduce the amount of oxygen in the weld metal, but even in the examples, the amount of oxygen is only reduced down to 334 mass ppm. The effect is not satisfactory.
In PLT 2, the deoxidizing elements Si, Mn, Mg, and Ti are added to restrict the total amount of oxides, including TiO2, and study the reduction of oxygen in the weld metal, but in the examples, only an effect of reduction down to 350 mass ppm is confirmed. The improvement is insufficient.
In PLT 3, use of restriction of the total amount of oxides, including TiO2, addition of CaF2, and addition of the deoxidizing elements Si, Mn, and Mg to reduce oxygen in weld metal is studied, but the amount of oxygen of the weld metal is 300 mass ppm or more. In this invention as well, the amount of oxygen of the weld metal cannot be sufficiently reduced.
Flux-cored wire includes, in addition to the rutile-based wire, wire using fluorides as main slag-forming materials (below, sometimes indicated as “fluoride-based FCW”). A fluoride-based FCW differs from a rutile-type FCW in that it enables easy reduction of the amount of oxygen of the weld metal, but has the issues that it is limited in welding position to flat or horizontal fillet and that vertical, overhead, and horizontal welding are extremely difficult. Specific case studies are for example disclosed by PLT's 4 to 7 which are listed below.
In PLT 4, fluorides of Ca, Ba, Mg, and Sr and carbonates and composite oxides of Ca, Ba, Mg, and Sr are studied as the main slag systems, but welding in a vertical, overhead, and horizontal position was not evaluated. Furthermore, the amount of oxygen of the weld metal and the results of evaluation of toughness are also not disclosed. That is, from PLT 4, no technical suggestions can be obtained at all on flux-cored wire with which all-position welding is possible and with which high toughness weld metal can be obtained.
In PLT 5, the content of the amounts of BaF2, Ni, Mn, C, Al, Si, and oxides is restricted to reduce the amount of oxygen of the weld metal down to 50 mass ppm, but this is predicated on welding by a direct current straight polarity using the welding wire as the negative electrode. With direct current straight polarity, lack of fusion easily occurs and the arc also becomes unstable, so the discovery which is disclosed here is also not sufficient.
In PLT 6, a flux which contains CaF2 and which contains composite oxides of alkali metals or alkali earth metals and Ti and Si is used to reduce the amount of oxygen of the weld metal down to 251 mass ppm, but there is no description of vertical, overhead, and horizontal welding. From PLT 6, no technical suggestions at all are obtained on flux-cored wire which enables all-position welding.
In PLT 7, BaF2 is added and the deoxidizing metal elements and other flux ingredients are adjusted to reduce the amount of oxygen of the weld metal, but only flat and vertical position welding are studied. Overhead welding and horizontal welding are not described at all. The content is insufficient for achieving all-position welding.
Further, in PLT 7, as an essential condition for enabling vertical welding, the content of Al is limited to 0.3% or more, so there is the issue that the weld metal deteriorates in toughness. In PLT 7, the deterioration in toughness cannot be completely avoided, so in a Charpy test, a 55 J or higher absorbed energy is deemed passing, but with a 55 J absorbed energy, there would be many structures which would not satisfy the design standard.
To obtain a high toughness weld metal, it is necessary to limit the Al content in the wire to less than 0.3%, but no technical suggestions at all can be obtained from PLT 7 on fluoride-based FCW which satisfies this requirement and enables welding in flat, vertical, overhead, and horizontal positions.
Summarizing the above, it is possible to conclude that there has not been flux-cored wire where a power source polarity using a direct current reverse polarity (polarity using welding wire as positive electrode) is possible, the content of Al can be restricted to a mass ratio with respect to the wire total mass of less than 0.3% to enable welding in the flat, vertical, overhead, and horizontal welding positions, oxygen of the weld metal can be stably suppressed to 300 mass ppm or less, and, as a result, the Charpy absorbed energy exceeds 200 J. In fact, no flux-cored wire which satisfies these characteristics has been commercialized up to the present.